1. Field of the Invention
This invention relates to an active matrix substrate and to a liquid crystal apparatus comprising such an active matrix substrate and liquid crystal for displaying pictures and characters as well as to a display apparatus comprising such a liquid crystal apparatus.
2. Related Background Art
The current world has entered a so-called multi-media age and devices and equipment for exchanging pictorial data for communication purposes are playing an ever-increasingly important role in the world. In this tide, liquid crystal apparatus are attracting attention because they can be realized in a thin and portable form and consume power only at a reduced rate so that the industry of manufacturing such devices has grown as one of the major industries that is comparable with the semiconductor industry.
Liquid crystal apparatus are at present popularly used for so-called notebook size personal computers having a size as short as ten inches. Additionally, liquid crystal is expected to play a major role in displays of electronic work stations and home television sets that typically comprise a large display screen. However, a large display screen involves high manufacturing cost and is required to meet rigorous electric requirements for driving such a large screen. As a rule of thumb, the manufacturing cost of a liquid crystal display panel is said to be proportional to the square or cube of the size of the screen.
Thus, as an alternative, a projection system designed to use a small liquid crystal panel and enlarge the image formed on it has been proposed. Such a system is made feasible mainly due to the recent development of finely sized semiconductor devices that operate excellently and can be manufactured at low cost.
In view of the recent technological development, there is an increasing demand for small TFTs provided with a satisfactory drive power to be used in liquid crystal display panels comprising TFTs. Additionally, TFTs using polycrystalline Si rather than amorphous Si are getting popularity. Video signals for the level of resolution meeting the NTSC standards that are used for the NTSC television system do not require high speed processing capabilities. Therefore, not only TFTs but also components of peripheral circuits including shift registers and decoders may be made of polycrystalline Si to produce a liquid crystal display apparatus wherein a display region and peripheral drive circuits are integrally formed.
However, polycrystalline Si is less performing than monocrystalline Si so that shift registers, for example, may have to be divided into a plurality of groups that are installed separately in order to realize a television set with a level of resolution higher than the level required by the NTSC standards or a computer display with the so-called XGA or SXGA class of resolution. Then, noise can appear as ghost along the boundaries of the separated devices in the display region to provide a problem to be solved.
In an attempt to bypass this problem, display apparatus comprising a monocrystalline Si substrate having a high drive potential have been proposed to replace display apparatus of the above described integral type using monocrystalline Si. Since the drive potential of the transistors of the peripheral drive circuits of such display apparatus is satisfactory, the above described technique of dividing devices is not necessary here and hence the noise problem can be successfully avoided.
However, it has been a tough challenge for known liquid crystal display apparatus to reduce the pixel size because the drive voltage of the liquid crystal is relatively high and hence the transistors have to show a correspondingly high withstand voltage. Additionally, there arises another problem of the difficulty of reducing the time constant of the signal lines because of an increase in the capacitance and the resistance of the signal lines if the lines and the wiring space of the apparatus are reduced, a problem that makes it difficult to realize a liquid crystal display apparatus comprising down-sized chips. Still additionally, there is another problem that a liquid crystal display apparatus normally show a large video line capacitance and requires a large drive potential for the external drive system, which is a problem that also baffles efforts for high speed operation and low manufacturing cost.